Conventional glass-ceramic articles were first disclosed in U.S. Pat. No. 2,920,971. As described therein, such articles are prepared by following three general steps: first, a glass forming batch normally containing a nucleating agent is melted; second, that melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass article of a desired geometry shaped therefrom; and third, that glass article is subjected to a heat treatment to effect crystallization in situ. That heat treatment is frequently conducted in two stages; viz., the glass article is initially heated to a temperature in the vicinity of or slightly above the transformation range to cause the development of nuclei therein, and thereafter the nucleated glass is heated to a temperature approaching or exceeding the softening point of the glass to promote the growth of crystals on those nuclei. That patent is incorporated herein by reference for its general discussion of the nature and production of conventional glass-ceramic articles.
There is no mention in that patent of glass-ceramics which are capable of being machined with common hand tools. Thus, conventional glass-ceramic articles tend to be brittle and to shutter when subjected to standard machining processes.
However, U.S. Pat. No. 3,689,293 disclosed compositions within the K.sub.2 O--MgO--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 --F system capable of forming machinable glass-ceramics due to the presence of fluormica crystals therein having a structure related to the natural mineral phlogopite. Other U.S. patents describing the preparation of machinable glass-ceramic articles include U.S. Pat. Nos. 3,732,087, 3,756,838, 3,801,295, and 3,997,352. Each of those patents discloses products containing a fluormica as the predominant crystal phase.
Spontaneously-formed glass-ceramic articles are described in several U.S. patents such as U.S. Pat. Nos. 3,985,531, 3,985,532, 3,985,533, 4,239,520, and 4,244,723. The method for producing such articles contemplates four general steps: a batch of the proper composition is melted; that melt is simultaneously cooled to a temperature about 100.degree.-300.degree. C. above the annealing point of the glass to develop phase separation and nucleation therein and a glass body of a desired configuration shaped therefrom; that glass body is exposed to a temperature between the annealing point of the glass and the temperature of phase separation and nucleation to cause crystallization on the nuclei; and the crystallized body is cooled to room temperature. Because the compositions crystallize so rapidly, the times of exposures to Steps 2 and 3 may be relatively brief. No further heat treatment is necessary.
U.S. Pat. No. 4,239,520 discloses the preparation of spontaneously-formed, machinable glass-ceramic bodies containing fluormica as the predominant crystal phase from compositions consisting essentially, in weight percent, of 6-11% K.sub.2 O, 1.7-4% Li.sub.2 O, 17-25% MgO, 50-65% SiO.sub.2 and 5-8.5% F.